RISC-V: Adding XTheadCmo ISA extension

This patch adds support for the XTheadCmo ISA extension.
To avoid interfering with standard extensions, decoder and translation
are in its own xthead* specific files.
Future patches should be able to easily add additional T-Head extension.

The implementation does not have much functionality (besides accepting
the instructions and not qualifying them as illegal instructions if
the hart executes in the required privilege level for the instruction),
as QEMU does not model CPU caches and instructions are documented
to not raise any exceptions.

Co-developed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230131202013.2541053-2-christoph.muellner@vrull.eu>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/meson.build

@@ -2,6 +2,7 @@
 gen = [
   decodetree.process('insn16.decode', extra_args: ['--static-decode=decode_insn16', '--insnwidth=16']),
   decodetree.process('insn32.decode', extra_args: '--static-decode=decode_insn32'),
+  decodetree.process('xthead.decode', extra_args: '--static-decode=decode_xthead'),
   decodetree.process('XVentanaCondOps.decode', extra_args: '--static-decode=decode_XVentanaCodeOps'),
 ]